7828 Density Transmitter Technical Manual Modbus Communications
78285000_AA D-13
D.6 ESTABLISHING MODBUS COMMUNICATIONS
Using ADView, it is possible to establish which devices are available on the network, and
their slave addresses. However, if you are not using ADView, the following procedure can
be adopted.
If the transducer Slave address or the values of Registers 47 and 48 are not known,
Modbus communications cannot be carried out successfully, and it will be necessary to
establish the current values in these items. The following procedure will do this.
The process is:
a) Find the slave address by trying all possible values until a response is received.
b) Establish whether the register size is 16 or 32 bits by reading register 48.
c) Find the byte order by reading register 47.
a) Make sure only the transducer is connected to the Modbus Master, then send the
following message (Read Register 47):
Slave
Address
Command Register
Address
Checksum
00 03 00 47
10
00 02 checksum
Wait for a response. If there is none, repeat the same message, with the Slave address
changed to 1, and await a response. Repeat the process until a response is obtained.
This will show the slave address of the transducer.
b) Send the following message (Read Register 48):
Slave
Address
Command Register
Address
Checksum
nn 03 00 48
10
00 02 checksum
where nn is the transducer's slave address.
The transducer will respond with either:
Slave
Address
Comman
d
Data Bytes Checksu
m
nn 03 04 4 data bytes checksum
showing that the transducer is set to 16 bits register size, or:
Slave
Address
Command Data Bytes Checksum
nn 03 08 8 data bytes checksum
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